The present invention relates to adhesive bonding laminates, and more particularly, to adhesive bonding laminates which may be used to attach a flexible circuit to a substrate. The invention is also directed to methods of attaching a flexible circuit to a stainless steel substrate, and to ink jet printhead assemblies and ink jet print cartridges including such adhesive bonding laminates.
Thermal ink jet print cartridges operate by rapidly heating a small volume of ink to generate a bubble caused by rapid vaporization of an ink vehicle for driving ink through one or more of a plurality of orifices so as to deposit one or more drops of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead moves across the paper. The thermal ink jet printer is generally fast and quiet, as only the ink droplet is in contact with the paper. Such printers produce high quality printing and can be made both compact and economical.
A typical ink jet cartridge assembly includes a cartridge body which is attached to a printhead assembly (sometimes referred to hereinafter as xe2x80x9cprintheadxe2x80x9d). Ink which is disposed within the cartridge body flows to the printhead and is expelled in a known manner. More particularly, the cartridge body includes a die cavity in which the printhead is disposed. The printhead is in the form of a nozzle plate attached to a semiconductor chip. A plurality of heater elements are carried by the semiconductor chip, with each heater element being disposed adjacent to a respective nozzle in the nozzle plate. An electrical circuit, which may be in the form of a TAB (Tape Automated Bonding) circuit, electrically interconnects the heater elements with appropriate circuitry in the inkjet printer such that the cartridge elements may be selectively energized as the carriage of the printer travels across the print medium.
The printhead is typically disposed within the die cavity of the cartridge body on a substrate. The silicon chip and nozzle layer are attached to the substrate using a known die attach adhesive. The TAB circuit typically surrounds the printhead and is fastened to the circuit platform of the cartridge using a pressure sensitive adhesive. The TAB circuit includes a plurality of copper leads which extend therefrom and connect with the heater elements on the printhead.
The flexible circuit, TAB or wire bond, is typically comprised of a polyimide layer on which copper conductive traces are formed. Preferably, a thin layer of gold is formed on top of the copper conductive traces and provides an amount of protection from inks which can cause corrosion of the circuits. However, for circuits used at higher voltages and higher operating speeds, an epoxy coating or the like is often screened on top of these gold/copper traces to give firther protection from inks. The circuit is typically adhered to the printhead and cartridge body by means of a pressure sensitive adhesive, for example an adhesive having an acrylic base.
During routine maintenance of the printhead in the printing process, the nozzle holes are wiped by a wiping mechanism. As a result, ink can travel to the underside of the circuit owing to capillary action of the acrylic-based pressure sensitive adhesive. The pressure sensitive adhesive allows the ink to wick under and be trapped between the circuit and the substrate, causing corrosion or electrical shorting of the flexible circuit in areas with no protective coating. Accordingly, there is a need for improved adhesives which exhibit a reduced ink wicking tendency.
When wire bonding, t he material use d to bond the circuit to the substrate must also provide the necessary support for the wire bond. The typical pressure sensitive adhesive is very soft and compliant and can yield under a load, and does not provide the stiffness under the flexible circuit that is essential for good wire bonding. Accordingly, an adhesive that has increased rigidity at a thickness of about 0.001 inch to about 0.003 inch is desired.
Furthermore, in many applications, precise alignment between the circuit and the print chip are necessary. In these cases, the circuit is in very close proximity or even in soft contact with the substrate during the alignment. The typical pressure sensitive adhesive does not allow the circuit to slip for fine alignment adjustments once it is in contact with the adhesive. One alternative to increase manufacturing efficiency is to provide a release liner on the pressure sensitive adhesive until after alignment and electrical inter connection of the flexible circuit and semiconductor chip. This however is not a reliable process, as removing the release layer potentially can stress the interconnect bonds between the flexible circuit and print chip where the release layer must be removed very close to any TAB bonds. Another alternative is to insert the adhesive between the circuit and substrate after electrically interconnecting the circuit and the print chip. However, the thin (about 0.001 inch) pressure sensitive adhesive bonding films are often too soft and compliant to be inserted between two surfaces in soft contact.
It would be advantageous to provide a film adhesive which provides good bonding, for example adhering to both a flexible circuit having an epoxy coating and a stainless steel substrate, while resisting ink wicking and allowing precise alignment of parts to be bonded. Therefore, there remains a need for an adhesive to attach a flexible circuit to a substrate, for example in an ink jet printhead assembly, which overcomes disadvantages of the prior art.
Accordingly, it is an object of the present invention to provide novel adhesive bonding laminates which overcome one or more disadvantages of the prior art. It is a more specific object of the invention to provide a dual layer adhesive bonding laminate which, when employed in an ink jet printhead assembly to bond a flexible circuit to a substrate, reduces or eliminates ink from wicking between the circuit and substrate and therefore prevents the ink from causing corrosion or electrical shorting of the flexible circuit.
These and additional objects and advantages are provided by an adhesive bonding laminate, ink jet printheads and ink jet print cartridges containing the same, and methods of attaching a flexible circuit to a substrate using the adhesive bonding laminate.
The adhesive bonding laminate of the present invention comprises a first adhesive film that is capable of adhesively bonding to an epoxy coating and a second adhesive film that is capable of adhesively bonding to a stainless steel substrate. Furthermore, the first adhesive film is adhesively bonded to the second adhesive film. In one embodiment of the invention, the first adhesive film comprises a polycarbonate adhesive film or a polyetherimide adhesive film, and the second adhesive film comprises a polyurethane adhesive film, a phenolic butyral adhesive film or a polyester adhesive film. Adhesive bond as used herein refers to a non-releasable, non-repositionable adhesive bond, unlike the releasable, repositionable adhesive bonds similar to those used as the adhesives in self-stick removable note pads, and the like.
The ink jet printhead of the present invention comprises a stainless steel substrate. A silicon chip is arranged on the stainless steel substrate. A flexible circuit or having an epoxy coating on one side thereof is attached to the stainless steel substrate using an adhesive bonding laminate of the present invention. The adhesive bonding laminate is located between the epoxy coated side of the flexible circuit and the stainless steel substrate.
The ink jet print cartridge of the present invention comprises an ink jet ink housing and an ink jet printhead, wherein the ink jet printhead comprises a stainless steel substrate attached to the ink jet print housing, a silicon chip located on the stainless steel substrate, a flexible circuit, and an adhesive bonding laminate of the present invention. The flexible circuit has an epoxy coating on one side thereof. The adhesive bonding laminate is located between the epoxy coated side of the flexible circuit and the stainless steel substrate.
The present invention also comprises a method for attaching a flexible circuit to a stainless steel substrate utilizing the adhesive bonding laminate of the present invention. A flexible circuit is aligned with a silicon chip attached to a stainless steel substrate. The alignment of the flexible circuit to the silicon chip preferably comprises aligning the wire bond pads of the flexible circuit with the bond pads of the silicon chip. Once aligned, the flexible circuit is then adhesively bonded to the stainless steel substrate using the adhesive bonding laminate. The rigidity of the adhesive bonding laminate enables the wire bond to be made. In one embodiment of the invention, the flexible circuit is electrically bonded to the silicon chip, and then the adhesive bonding laminate is inserted between the flexible circuit and the stainless steel substrate. Electrically bonding comprises making an electrical connection between the flexible circuit and the silicon chip. The flexible circuit is then adhesively bonded to the stainless steel substrate using the adhesive bonding laminate.
Still other objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the following detailed description, which is simply by way of illustration various modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different obvious aspects all without departing from the invention. Accordingly, the drawings and the description are illustrative in nature and not restrictive.